Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving member. The ink can be stored in reservoirs that are located within cartridges installed in the printer. Such ink can be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the image receiving surface. In these solid ink printers, also known as phase change inkjet printers, the solid ink can be in the form of pellets, ink sticks, granules, pastilles, or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device, which melts the solid ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. Other inkjet printers use gel ink. Gel ink is provided in gelatinous form, which is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead. Once the melted solid ink or the gel ink is ejected onto the image receiving member, the ink returns to a solid, but malleable form, in the case of melted solid ink, and to a gelatinous state, in the case of gel ink.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving member having an image receiving surface to form an ink image during printing. The image receiving surface can be the surface of a continuous web of recording media, a series of media sheets, or the surface of an image receiving member, which can be an imaging drum, a rotating print drum, or an endless belt. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an aperture, usually called a nozzle, in a faceplate of the printhead. The actuators expel an ink drop in response to an electrical signal, sometimes called a firing signal. The magnitude, or voltage level, of the firing signals affects the amount of ink ejected in an ink drop. The firing signal is generated by a printhead controller with reference to image data. A print engine in an inkjet printer processes the image data to identify the inkjets in the printheads of the printer that are operated to eject a pattern of ink drops at particular locations on the image receiving surface to form an ink image corresponding to the image data. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to electronic image data.
Phase change inkjet printers form images using either a direct or an offset print process. In a direct print process, melted ink is jetted directly onto recording media to form images. In an offset print process, also referred to as an indirect print process, melted ink is jetted onto a surface of a rotating member such as the surface of a rotating drum, belt, or band.
Indirect inkjet printers are capable of producing either simplex or duplex prints. Simplex printing refers to production of an image on only one side of a print media. Duplex printing produces an image on each side of a media sheet. In duplex indirect printing, an ink image is initially formed on a rotating drum and then transferred to the media. The media sheet is then inverted and sent along a path that passes the second side of the media sheet by the rotating drum upon which the ink has been deposited for the formation of a second ink image on the second side.
Recording media are heated and are moved proximate the surface of the rotating member in synchronization with the ink images formed on the surface. The recording media are then pressed against the surface of the rotating member as the media passes through a nip formed between the rotating member and a transfix roller. The ink images are transferred and affixed to the recording media by the pressure in the nip. This process of transferring an image to the media is known as a “transfix” process.
The nip is maintained at a high pressure by forcing a high durometer synthetic transfix roller against the rotating member. As the rotating member rotates, the recording media is pulled into and through the nip and is pressed against the deposited ink image by the opposing surfaces of the transfix roller and the rotating member. The high pressure conditions within the nip compress the media and ink together, spread the ink droplets, and fuse the ink droplets to the media. Heat from the preheated media heats the ink in the nip, making the ink sufficiently soft and tacky to adhere to the print media. When the print media leaves the nip, stripper fingers or other like members peel it from the printer member and direct it into a media exit path.
Increased printing speeds can be achieved by increasing the rotational speed of the imaging drum or by increasing the diameter of the imaging drum. If the diameters are increased, the width of the nip increases. In addition, as print speed increases, higher pressures are required at the nip, which also increases the width of the nip. Consequently as print speeds increase, the shape and size of the nip can affect print conditions.
Because the application of the high pressures needed for high speed imaging results in deformation of the transfix roller, the shape of the transfix roller can affect the shape and size of the nip as well. In some printers, a transfix roller having a “crowned profile” can be used to provide a desired nip and nip width. A “crowned profile” is a profile wherein the diameter of the transfix roller located at the middle of the roller is larger than the diameter of the transfix roller located at the ends of the roller. Transfix rollers with a crowned profile can provide a desired image quality, roller life, and acceptable cost. In other printers, a transfix roller having a flat profile can be used.
A nip typically includes a length defined by the length of the transfix roller and the force of contact between the transfix roller and the image receiving member. For instance in a transfix roller having a crown, the length of the nip can be shorter than the length of the roller. The width of the nip, which is measured in the process direction is defined by the pressure applied between the transfix roller and the image receiving member and the materials comprising the transfix roller and the image receiving member.
The transferred ink drops should spread out to cover a specific area to preserve image resolution. Too little spreading leaves gaps between the ink drops while too much spreading results in intermingling of the ink drops. Additionally, the nip conditions should be controlled to maximize the transfer of ink drops from the image receiving member to the print media without compromising the spread of the ink drops on the print media. Moreover, the ink drops should be pressed into the paper with sufficient pressure to fix the ink drops to the paper. Otherwise, the ink drops can be inadvertently removed by abrasion resulting in poor image quality. Therefore, to optimize image resolution, the conditions within the nip should be carefully controlled.